Diabetic neuropathy (DN) is a significant cause of disability. The Diabetes Complications and Control Trial suggests DN is most responsive when treated early. We find neuropathy is associated with early glucose dysregulation. Neuropathy associated with diabetes and impaired glucose tolerance (IGT) injures small nerve fibers first. Consequently, traditional progression measures such as vibration detection threshold and nerve conduction studies, which measure large fiber function, detect early DN poorly. The lack of sensitive measures of small fiber loss limits study of potential therapies when they have the greatest chance of success. Skin biopsy with measurement of intraepidermal nerve fiber density (IENFD) is a sensitive, quantitative and reproducible measure of small fiber injury, but longitudinal data correlating IENFD with clinical neuropathy progression is not available. Early microvascular injury is important in the pathogenesis of DN. Metabolic consequences of hyperglycemia (activation of the polyol pathway, formation of advanced glycation endproducts and reactive oxygen species) impair release of the vasodilator nitric oxide and cause endothelial injury and nerve ischemia. Laser Doppler is a sensitive means of measuring change in cutaneous microvascular blood flow. Patients with DN display reduced cutaneous vasodilatation after iontophoresis of acetylcholine, consistent with a deficit in nitric oxide function. We hypothesize: (1) IENFD will be a sensitive measure of early DN progression, (2) cutaneous laser Doppler flow deficits will correlate with IENFD decline (3) and will predict development of clinically defined DN. We will establish the sensitivity of IENFD to DN progression and correlate change with traditional endpoint measures. A group of diabetic subjects without neuropathy will be followed in order to determine if baseline IENFD, or rate of change in IENFD over one year, predicts future development of clinical neuropathy. IENFD will be correlated with microvascular dysfunction using laser Doppler flow following iontophoresis of acetylcholine. We will determine if microvascular changes precede small fiber nerve loss or predict nerve loss over time. Microvascular changes and IENFD will be correlated with glycemic control. The results will aid in the design of future therapeutic trials in DN and provide important information regarding the role of microvascular dysfunction in the pathogenesis of diabetic small fiber neuropathy.